Merge branch 'issue/document_gradtransfer' into 'develop'

Added some documentation to the GridTransferInterface and GridTransfer

See merge request !101
4 jobs for develop in 47 minutes and 41 seconds (queued for 35 seconds)
Status Job ID Name Coverage
  Test
passed #8793
debian:10 clang-6-libcpp-17

00:25:16

passed #8792
debian:10 gcc-8-17

00:17:18

passed #8794
debian:9 gcc-6-14

00:18:40

passed #8795
ubuntu:18.04 clang-6-17

00:23:18